Field of the Disclosure
The present disclosure relates to a non-reciprocal circuit element, and particularly relates to a non-reciprocal circuit element such as an isolator or a circulator used in the microwave bands.
Description of the Related Art
Hitherto, a non-reciprocal circuit element such as an isolator or a circulator has characteristics of transmitting signals only in a predetermined specific direction and not transmitting signals in the opposite direction. By utilizing the characteristics, for example, the isolator is used in a transmitting circuit unit of a mobile communication apparatus such as a cellular phone.
As such a type of non-reciprocal circuit element, Patent Document 1 discloses a non-reciprocal circuit element in which two center electrodes 21 and 22 (inductance elements L1 and L2) formed on a ferrite 10 and capacitors C1 and C2 form two LC parallel resonant circuits, and a resistance element R is connected in series between ends of the respective center electrodes 21 and 22, as shown in FIG. 11.
In the non-reciprocal circuit element disclosed in Patent Document 1, when a high-frequency signal is inputted to an input terminal IN (a forward direction), ports P1 and P2 are in-phase, and no current flows through the resistance element R, so that insertion loss decreases. Meanwhile, when a high-frequency current is inputted to an output terminal OUT (a reverse direction), a phase difference occurs between the potentials at the ports P1 and P2, and a current flows through the resistance element R, so that the high-frequency current is greatly attenuated (isolation characteristics are obtained).
However, since a frequency region of a high-frequency signal where the phase difference between the potentials at the ports P1 and P2 is great is limited, it is difficult to obtain preferable isolation characteristics over a wide band.
In addition, Patent Document 2 indicates that the band in which isolation characteristics are obtained is widened by connecting an LC series resonant circuit to a resistance element. In the technique disclosed in Patent Document 2, the band in which isolation characteristics are obtained is widened by matching a resistance element over a wide range on the basis of the resonance characteristics of the LC series resonant circuit.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-237613
Patent Document 2: Japanese Patent No. 4155342